A series of variants of HSV which have altered patterns of pathogenesis in experimental animal systems have been identified. Examples include: 1) HSV-I and II intertypic recombinants (RE6 and RS6) which are non-neurovirulent when injected into adult mouse brains. 2) A strain of HSV-I (KOS) which is nonvirulent in mice when injected in foot pads. 3) A revertant of a ts mutant of HSV-I (tsI) which is not temperature sensitive for replication in non-nervous tissues, but which does not reactivate from the latent state in spinal ganglia at 38.50C. In addition, the inability of HSV-I to invade mesoderm in the CAM of embryonic chicks is being investigated. In all cases, trivial explanations for the altered pathogenicity have been rigorously excluded. Data are presented demonstrating that pathogenesis can be rescued using DNA transfections of wild-type parental DNA restriction fragments (molecularly cloned) and intact variant virus DNA followed by selection in vivo. A combined collaborative study of the genes and gene products involved in these and related examples of HSV pathology and neurovirulence using techniques of both molecular biology and pathobiology is described.